A fuel level probe with a series of stacked capacitors is described in US-A-2012/0065904. In one embodiment (FIG. 2) the electrodes are annular and in other embodiments (FIGS. 6A and 6B) the electrodes are planar. The capacitance of each of the capacitive sections is measured sequentially and then used to infer the level of fuel and/or water in an aircraft fuel tank.